Parkinson's disease is characterized pathologically by selective degeneration of dopamine-containing neurons in the substantia nigra pars compacta (SNc). The etiology in the vast majority of individuals with the disorder remains elusive but ageing is an important risk factor. Nigral cell death in PD is accompanied by the accumulation of oxidatively damaged proteins, aggregation of proteins and the formation of proteinaceous intracytoplasmic Lewy body inclusions. These observations suggest that failure of the ubiquitin-proteasome system (UPS), the biochemical pathway primarily responsible for the degradation of abnormal and short-lived regulatory/transcriptional proteins may underlie nigral pathology in Parkinson's disease. Indeed, mutations in the genes encoding alpha-synuclein and 2 enzymes of the UPS, namely parkin and ubiquitin Cterminal hydrolase L1, are associated with altered protein handling in rare familial forms of Parkinson's disease. However, these or similar gene defects do not occur in most patients who have sporadic Parkinson's disease. We hypothesize that defects in 26/20S proteasomes cause the UPS to fail and this underlies protein accumulation, Lewy body formation and dopaminergic neuronal death in the SNc in sporadic Parkinson's disease. Consistent with this hypothesis, our preliminary findings demonstrated structural and function defects in 26/20S proteasomes, and a several-fold increase in the levels of poorly degraded/undegraded and potentially cytotoxic ubiquitinated protein substrates, in the SNc but not elsewhere in sporadic Parkinson's disease. In addition, we showed that in aged control subjects and adult rats, dopaminergic neurons of the SNc have relatively low 26/20S proteasomal activity and poor expression of the proteasome activators (PA28 and PA700) compared to other brain regions. Further, we showed that inhibition of 26/20S proteasomal function causes selective degeneration of dopaminergic neurons with the formation of alpha-synuclein/ubiquitin-immunoreactive inclusions in primary mesencephalic cultures and in rat SNc with motor dysfunction. In this project, we propose to determine (1) if and how the structure and function of proteasomes are defective in all stages of sporadic PD; (2) if low proteasomal function normally occurs in the SNc of controls as this may underlie its selective vulnerability and degeneration in PD; (3) if proteasomal dysfunction underlies Lewy body formation; and (4) if proteasomal dysfunction plays a role in nigral dopaminergic cell death in sporadic Parkinson's disease. These studies will test our hypothesis that inadequate proteasomal function underlies both vulnerability and degeneration of the SNc in sporadic Parkinson's disease.